Casting molds are essentially made up of molds or molds and cores which represent the negative shapes of the castings to be produced. Said cores and molds consist of a refractory material, for example quartz sand, and a suitable binder that imparts adequate mechanical strength to the casting following removal from the mold. The refractory mold base material is preferably present in a free-flowing form, so that it can be packed into a suitable mold cavity and compressed there. The binder produces firm cohesion between the particles of the mold base material, so that the casting mold achieves the required mechanical stability. In casting, molds from the outer walls for the casting, and cores are used to produce cavities within the casting. It is not absolutely necessary for molds and cores to be made of the same material. For example, in chill casting the shaping of the outer area of the casting is formed using metal permanent molds. A combination of molds and cores produced from mold mixtures of different compositions and using different methods is also possible. If only the term “molds” is used in the following for the sake of simplicity, the statements apply equally for cores as well which are based on the same mold mixture and produced according to the same method.
Molds can be produced using both organic and inorganic binders which may be cured by either cold or hot methods in each case.
The cold method is the name applied to methods which are performed essentially without heating the molding tool, generally at room temperature or at a temperature adequate for producing a reaction if desired. For example the curing is performed in that a gas is passed through the mold material mixture to be cured and produces a chemical reaction at this time. In hot methods the mold material mixture, after molding, for example, is heated by the hot molding tool to a sufficiently high temperature to expel the solvent present in the binder and/or to initiate a chemical reaction for curing the binder.
Because of their technical characteristics, organic binders have great financial significance on the market at the present time. Regardless of their composition, however, they have the drawback that they decompose during casting, thereby emitting considerable quantities of harmful materials such as benzene, toluene and xylenes. In addition the casting of organic binders generally leads to odor and fume nuisances. In some systems harmful emissions even occur during the manufacturing and/or storage of cores. Even though the emissions have been reduced gradually over the years by binder development, they cannot be completely avoided with organic binders. For this reason, in recent years research and development activity is again turning toward inorganic binders in order to improve them and the product properties of the molds and cores produced with them.
Inorganic binders have long been known, especially those based on the water glasses. They found their broadest use during the 1950s and 1960s, but they rapidly lost their significance with the emergence of modern organic binders. Three different methods are available for curing the water glasses:                passing a gas, for example CO2, air or a combination of the two, through them,        addition of liquid or solid curing agents, for example esters        thermal curing, e.g., in the hot box method or by microwave treatment.        
CO2 curing is described, for example, in GB 634817; curing with hot air without added CO2 for example in H. Polzin, W. Tilch and T. Kooyers, Giesserei-Praxis 6/2006, p. 171. A further development of CO2 curing by subsequent flushing with air is disclosed in DE 102012103705.1. Ester curing is known for example from GB 1029057 (so-called No-Bake method).
The thermal curing of water glass is discussed for example in U.S. Pat. No. 4,226,277 and EP 1802409, wherein in the latter case particulate synthetic amorphous SiO2 is added to the mold material mixture to increase the strength.
Other known inorganic binders are based on phosphates and/or a combination of silicates and phosphates, wherein the curing is likewise performed according to the above-mentioned methods. The following may be mentioned in this connection as examples: U.S. Pat. No. 5,641,015 (phosphate binders, thermal curing), U.S. Pat. No. 6,139,619 (silicates/phosphate binders, thermal curing), U.S. Pat. No. 2,895,838 (silicate/phosphate binders, CO2 curing), and U.S. Pat. No. 6,299,677 (silicate/phosphate binders, ester curing).
In the cited patents and applications EP 1802409 and DE 102012103705.1 it is suggested that amorphous silica be added to each of the mold material mixtures. The SiO2 has the task of improving the breakdown of the cores after exposure to heat, for example after casting. In EP 1802409 B1 and DE 102012103705.1 it is illustrated extensively that the addition of synthetic particulate amorphous SiO2 brings about a distinct increase in strength.
It is suggested in EP 2014392 B1 that a suspension of amorphous spherical SiO2 be added to the mold material mixture, consisting of mold material, sodium hydroxide, alkali silicate-based binder and additives, wherein the SiO2 should be present in two particle size classes. By this means good flowability, high bending strength and a high curing speed would be obtained.